<?php
abstract class Person{
	private $age;
	public abstract function show_age();
}
class Student extends Person{
	function __construct($age)
	{
		$this->age = $age;
	    //Person::$age = $age;
	}
	public function show_age()
	{
		echo "$this->age \n";
	}
}
$s=new Student(31);
$s->show_age();
print_r($s);